Support SNI and multiple SSL certificates

Rebasification and pull-requestification of https://github.com/fblee/EventMachine-LE/compare/d5a384e...wip.sni-multiple-certs.

Reference: http://stackoverflow.com/questions/5113333/how-to-implement-server-name-indicationsni-on-openssl-in-c-or-c-are-there-a